A conventional filter manufacturing machine includes a storage vessel storing acetate tow for use as filter fibers, for example, and the tow (filter fibers) is fed from the storage vessel along a predetermined feed path. In the process of feeding, the tow is opened, spread, and formed into a sheet material. Subsequently, a plasticizer such as triacetin is added to the sheet material, and the sheet material is supplied to a rod forming device. In the rod forming device, the sheet material is formed into a rod, and the rod-shaped material is wrapped in paper to continuously form a filter rod.
The sheet material is formed into a rod by means of a part called forming tube. While passing through the forming tube, the sheet material is compressed and formed into a rod. There has been known a conventional technique whereby the sheet material is formed into a hollow cylindrical rod while passing through the forming tube.
In order to make a through hole in the rod, a mandrel is previously arranged inside the forming tube along the axis of the forming tube. The sheet material introduced into the forming tube is formed into a rod around the mandrel, continuously delivered, and pulled out of the mandrel, whereupon the rod with a through hole is obtained. It is, however, often the case that the tow projects from the surface of the through hole, and a filter rod with such a through hole is regarded as low in quality and rejected as a defective article. Also, if the surface region of the through hole of the rod is not satisfactorily high in hardness, the through hole is crushed when the filter rod is cut in a subsequent process, leading to lowering of the quality. In order to prevent crushing of the through hole, the amount of the plasticizer used may be increased to thereby increase the hardness of the filter. If the plasticizer is used in a large amount, however, a problem arises in that the filter itself is dissolved or gives off an offensive smell.
On the other hand, Patent Document 1 discloses an apparatus for manufacturing a hollow cylindrical filter rod. In this manufacturing apparatus, a tube is arranged along the axis of a filter rod to be formed, and tow is gathered around the tube and hardened to form a hollow filter. The apparatus, however, requires an additional member, namely, the tube, which leads to increase in the number of component parts. Also, a tube feed mechanism needs to be additionally provided, making the apparatus and its control complex.